To maximize performance and communication capacity of a wireless communication system, a multiple input multiple output (MIMO) system has drawn attention in recent years. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas to improve transfer efficiency of data to be transmitted or received. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then collected as one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can be increased with respect to a specific data transfer rate.
The MIMO technique includes transmit diversity, spatial multiplexing, and beamforming. The transmit diversity is a technique in which the multiple Tx antennas transmit the same data so that transmission reliability increases. The spatial multiplexing is a technique in which the multiple Tx antennas simultaneously transmit different data so that data can be transmitted at a high speed without increasing a system bandwidth. The beamforming is used to add a weight to multiple antennas according to a channel condition so as to increase a signal to interference plus noise ratio (SINR) of a signal. In this case, the weight can be expressed by a weight vector or a weight matrix, which is respectively referred to as a precoding vector or a precoding matrix.
The spatial multiplexing is classified into single-user spatial multiplexing and multi-user spatial multiplexing. The single-user spatial multiplexing is also referred to as a single user MIMO (SU-MIMO). The multi-user spatial multiplexing is also referred to as a spatial division multiple access (SDMA) or a multi user MIMO (MU-MIMO). A capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel can be decomposed into independent channels. If the number of Tx antennas is Nt, and the number of Rx antennas is Nr, then the number of independent channels is Ni where Ni≦min{Nt, Mr}. Each independent channel can be referred to as a spatial layer. A rank represents the number of non-zero eigen-values of the MIMO channel and can be defined as the number of spatial streams that can be multiplexed.
For the purpose of data transmission/reception, system synchronization acquisition, channel information feedback, etc., there is a need to estimate an uplink channel or a downlink channel in the wireless communication system. Channel estimation is a process of recovering a transmission signal by compensating for signal distortion in an environment where a rapid change occurs due to fading. In general, channel estimation requires a reference signal or a pilot known to both the transmitter and the receiver.
In the multi-antenna system, each antenna may experience a different channel, and thus there is a need to design a deployment structure of a reference signal by considering each antenna. Conventionally, when a signal is transmitted from a base station to a user equipment, reference signals are deployed under the assumption that up to 4 antennas are used. However, a next generation wireless communication system can transmit a downlink signal by using a more number of antennas, i.e., up to 8 antennas. In this case, how to deploy and transmit the reference signals needs to be taken into account.